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Chaste trees (Vitex agnus-castus) and butterfly bush (Buddleia davidii) both have terminal clusters of fragrant, small tubular flowers that attract butterflies. Suitable for butterfly gardens, the plants are deciduous and have similar cold hardiness, growing in U.S. Department of Agriculture plant hardiness zones 6 through 9. The plants are not closely related. Chaste tree is in the mint family and butterfly bush belongs in the snapdragon family. Plants differ in a number of respects.

Growth Habit

Chaste tree grows as a broadly spreading large shrub to small tree, reaching 15 to 25 feet tall and wide. Trees have multiple or single trunks. Use it as a specimen plant or in borders. Butterfly bush is a large shrub that varies in size depending on the cultivar, but the usual size is 6 to 8 feet tall and 3 to 5 feet wide. Plants grow and flower better if the branches are cut back to the ground every year in late winter. It is best grown massed in borders, butterfly gardens or cottage gardens rather than as a single specimen.

Leaves

Chaste tree has compound leaves of five to seven lance-shaped leaflets, each leaflet about 6 inches long, giving a rather lacy effect. Aromatic leaves are dark-green above, with the undersides a lighter color and hairy. When grown as a tree, it furnishes moderate shade. Butterfly bush has single leaves 10 to 12 inches long, green above and white or gray underneath, densely covering the shrub in summer.

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Flowers

Usually with purple flowers, varieties of chaste tree have pink or white flowers, with one variety (Vitex agnus-castus var. latifolia) having bright blue flowers. Flower clusters are about 7 inches long, drooping from the ends of branches in early summer. Flower clusters of butterfly bush are from 4 to 10 inches long, displayed to advantage at the ends of the long, arching branches. Bloom is from summer into fall. Butterfly bush flowers are suitable for cut flowers, while chaste tree does not lend itself to this use.

Fruits

Also called monk's pepper from the appearance and aroma of the seeds, chaste tree produces a fleshy fruit with four black seeds that are sometimes used for seasoning. Butterfly bush fruits are small, elongate capsules less than 1/2 inch long, green at first but then turning brown. The capsule's two compartments hold numerous small winged seeds.

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About the Author

Cathryn Chaney has worked as a gardening writer since 2002. Her horticultural experience working in the nursery industry informs her garden articles, especially those dealing with arid landscaping and drought-tolerant gardening. Chaney also writes poetry, which has appears in "Woman's World" magazine and elsewhere. Chaney graduated from the University of Arizona in 1992 with a Bachelor of Arts in English.